1. Field of the Invention
The present invention relates to an electrophotographic image forming technology, and more particularly to an image forming method, and an image forming apparatus and a process cartridge downsizable stably producing quality images without foggy or hazy images even when repeatedly producing images at lower cost and high speed.
2. Discussion of the Background
Low-end laser beam printers are recently being produced at lower cost, downsized and having higher printing speed to comply with expanding demands from SOHO (Small Office/Home Office) and consumers. Particularly, each part and device of the printers are required to be smaller and simplified for downsizing and lower cost of the printers. For example, charging methods are not exceptional. Corona charging methods are used for conventional electrophotographic image forming methods. However, the corona chargers needing high-voltage electric power are expensive and produce ozone harmful to environment. Therefore, inexpensive and small charging rollers are recently being used.
Charging rollers classified into two types contacting an image bearer and not contacting an image bearer. It is difficult to design the non-contact charger having a fixed discharge gap and it needs high voltage, which increases the cost.
Meanwhile, the contact charger is capable of controlling a discharge gap with a nip thickness, and easier to design and downsize than the non-contact charger.
However, the contact charger is likely to have an influence of a noise on an image bearer, i.e., contamination such as a toner remaining thereon after cleaned and a free external additive, resulting in contamination of charging roller and poor charging. The toner is removable with a cleaning member such as a nonwoven fabric and a brush, but the external additive free from the toner is difficult to remove. When the charging roller has a small surface roughness, e.g., not greater than Rz 5, the uniformity of the surface improves and uneven charging seems prevented. However, the charging roller and the image bearer slip each other actually, and particularly when a contact pressure therebetween is low, e.g., not greater than 4, they slip more each other and the image bearer is unevenly charged. When the surface roughness is larger than predetermined, e.g., not less than Rz 20, the surface uniformity is insufficient and the image bearer is unevenly charged.
In low-cost small printers, the one-component developing method without using a carrier is typically used more than the conventional two-component developing method. Even the one-component developing method includes the above-mentioned contact and non-contact charging methods, and the contact charging method is preferably used.
silicon oxide and titanium oxide are mainly used as external additives for use in a toner for the one-component developing method. The external additives peel off from the toner due to a stress between a developing roller and the image bearer and a stress when the image bearer is cleaned, resulting in contamination of the charging roller. Particularly when the charging roller is contaminated with titanium oxide, the electrical properties of the charging roller, i.e., the chargeability thereof is impaired, resulting in image quality problems. However, it is difficult to control chargeability with only silicon oxide without using titanium oxide. Particularly, it is difficult to prevent production of hazy images caused by exaggerated feeding amount of a toner on the developing roller due to increase of charge amount in an environment of low temperature and low humidity.
Various methods of improving the cleanability of a toner are studied.
Japanese published unexamined application No. 2004-177747 discloses a toner for developing electrostatic latent images, formed of an external additive including a particulate core-shell formed silica-coated metal oxide including a core layer formed of a member selected from the group consisting of titanium dioxide, aluminum oxide and zinc oxide, and a shell layer formed of silica and a particulate silica having a volume-average particle diameter of from 5 to 20 nm; and a particulate colorant.
Japanese published unexamined application No. 2002-182424 discloses a toner capable of producing images having high density without producing foggy or hazy images and filming even when continuously producing images for a long time, formed of a particulate colorant (and a binder resin); and an external additive including a particulate core-shell formed silica-coated metal oxide including a core layer formed of a member selected from the group consisting of titanium dioxide, aluminum oxide and zinc oxide, and a shell layer formed of silica and a particulate silica having an average particle diameter of from 10 to 30 nm and sphericity of from 1 to 1.3.
However, neither of the above-mentioned applications specifies a ratio of the core and shell, and the external additive has a small particle diameter and is likely to be freed from the toner. Such a toner is difficult to avoid contaminating a charging roller.
Japanese published unexamined application No. 2004-233407 discloses a method of specifying a surface potential after testing the durability of a toner layer on a developing roller to collect a toner remaining on a photoreceptor after transferring a toner image.
Japanese published unexamined application No. 2003-043785 discloses a method of forming an electrostatic latent image on a latent image bearer with a charging roller uniformly charged, which is formed of a metallic core material coated with a rubber layer having a surface roughness of from 0.5 to 10 μm, and which is further coated with a polyurethane resin having a glass transition temperature of from 30 to 80° C.; and developing the electrostatic latent image with a toner including boron or phosphorus in an amount of from 0.1 to 100 ppm to form a toner image for preventing contamination of the charging roller and foggy images. Namely, the born and phosphorus included in the toner prevents the toner from scattering, producing foggy images and filming.
However, the above-mentioned method cannot completely remove contamination due to an external additive free from the toner, and particularly when titania (titanium dioxide) is used as an external additive, the charging roller is more contaminated.
Japanese published unexamined application No. 2006-220765 discloses a method of cleaning a residue such as a toner or an external additive adhering to a charge with an image developer or an image bearer of an image forming apparatus.
However, this costs much and needs additional parts, resulting in difficulty of meeting lower reduction and downsizing. Particularly, this is not applicable to small printers.
As mentioned above, a charging roller having a surface roughness Rz of from 5 to 20 enables the charging roller and an image bearer to be driven by each other and the image bearer can uniformly be charged. A charging roller simply having such a surface roughness Rz scrapes a toner on an electrostatic latent image bearer and an external additive free therefrom worsens contamination (filming) of the charging roller. Silicon oxide and titanium oxide are mainly used as an external additive for a toner (for a one-component developing method). When a charging roller is contaminated with titanium oxide, the chargeability thereof is impaired, resulting in image quality problems. When only the silicon oxide is used without using the titanium oxide, the charging roller is difficult to have chargeability, resulting in difficulty of preventing production of abnormal images.
Because of these reasons, a need exists for an image forming method capable of meeting lower cost, downsizing and high-speed printing, preventing contamination of a contact charging roller due to a toner and its external additives even when repeatedly used for long periods, stably producing quality images without production of foggy and hazy images with a contact charging method (a charging roller) preventing a toner from being charged too much even in an environment of low temperature and low humidity.